The trend in the magnetic recording industry is towards higher density recording. Higher density recording requires materials with improved signal output at shorter wavelengths. Higher density recording also requires materials in which self-demagnetization can be minimized as much as possible.
One material that shows great potential for high density recording applications is cobalt metal. A cobalt metal particle can have a platelet-shape and a close-packed, hexagonal crystallographic structure. Cobalt metal particles with a close-packed, hexagonal, platelet shape have an easy axis of magnetization that is perpendicular to the plane of the particle. Such cobalt metal particles have high saturation magnetization, over 100 emu/g, and coercivities of up to 2000 Oe.
Cobalt metal particles suitable for magnetic recording are obtained by the reduction of pink, hexagonal, platelet-shaped beta cobaltous hydroxide. Various methods for making the beta cobaltous hydroxide precursor are known in the art. In one typical method, beta cobaltous hydroxide is made by reacting a cobaltous salt in an alkaline solution to form a precipitate of beta cobaltous hydroxide. This direct precipitation method is described in each of the Japanese publications JP54-75597, JA60-263328, and JP61-163123.
Unfortunately, the direct precipitation method provides insufficient control for adjusting the average particle size of the beta cobaltous hydroxide reaction product beyond a very limited range. What was needed in the art was a method of making beta cobaltous hydroxide in which a desired average particle size could be easily obtained over a relatively broader size range, while at the same time maintaining uniform particle shape and a narrow particle size distribution.